A scanning probe microscope (Scanning Probe Microscope, hereinafter abbreviated as “SPM”) is a device in which a tip end of a tiny measurement probe (probe) is brought closer to a sample surface, and the sample surface is scanned with the probe while a mechanical or electrical interaction between the probe and the sample is being detected, thereby to observe the shape of the sample surface, the distribution of the electrical characteristics, and the like. In an atomic force microscope (Atomic Force Microscope, hereinafter abbreviated as “AFM”) which is a typical SPM, an interatomic force is measured as the interaction that acts between the probe and the sample surface (see, for example, Non Patent Literature 1).
An AFM is normally provided with a scanner that moves a sample in three axial directions of X, Y, and Z that are orthogonal to each other (here, it is assumed that the X-axis and the Y-axis are taken in a plane on which the sample is mounted, and the Z-axis is taken in the direction that is orthogonal to the plane), a cantilever that is disposed at a position distant from the sample in the Z-axis direction and that has a probe attached to the tip end, and a displacement detection unit for detecting the deflection of the cantilever, where the tip end of the probe is brought extremely close (with a spacing of about several nm or less) to the sample. At this time, an interatomic force (attractive force or repulsive force) acts between the probe and the sample. When the scanner is driven along the sample surface, that is, so that the probe and the sample move relative to each other in the two axial directions of X-axis and Y-axis while keeping this interatomic force constant, the cantilever is displaced in the Z-axis direction in accordance with the elevation of the sample surface. The amount of spacing is detected by the displacement detection unit, and the scanner is subjected to feedback control so as to finely move the sample in the Z-axis direction so that the spacing may be kept constant. The control amount for this feedback control reflects the elevation of the sample surface, so that this control amount is taken into the data processing unit and processed, thereby to prepare a sample surface image to be displayed.
There are several operation modes for the AFM. Typical modes are a contact mode and a non-contact (dynamic) mode. In the contact mode, a bend caused in the cantilever by the repulsive force that acts between the probe and the sample surface when the probe is brought close to the sample is detected, and the scanner is subjected to feedback control so that the amount of the bend be kept constant. On the other hand, in the dynamic mode, the cantilever brought close to the sample surface is vibrated at a frequency around its resonance point. The amplitude of vibration changes mainly by an attractive force that acts between the probe and the sample surface. The scanner is subjected to feedback control so that the amplitude of this vibration be kept constant. In either mode, the operation is common in that the sample surface image is created by using the control amount for the feedback control.
However, if a steep change in the elevation is present on a sample surface when the sample surface is scanned with the probe, tracking delay may be generated or the tracking cannot be made in the feedback control of the scanner, so that a temporary disturbance may be generated in the feedback control. In such a case, the image data obtained based on the control amount for the feedback control may become inaccurate, and does not reflect the actual shape of the surface of the sample.
Usually, the SPM is used in such a manner that, when the sample surface is scanned with the probe, the same one-dimensional region is scanned for two times, that is, in the forward direction and in the reverse direction (in other words, reciprocal scanning is made), so as to collect image data independently. The forward scanning is referred to as trace, whereas the reverse scanning is referred to as retrace. One of the big reasons for carrying out such reciprocal scanning is that there is a difference in the shape of the tip end of probe or in the spring constant of the cantilever depending on the scanning direction, so that, even when the sample surface is appropriately scanned, different image data may be obtained in the trace and in the retrace. For this reason, a sample surface image is often created by taking an average of the image data obtained in the trace (which is hereinafter referred to as “trace image data”) and the image data obtained in the retrace (which is hereinafter referred to as “retrace image data”).
In the SPM disclosed in Patent Literature 1, characteristic data processing is carried out using the trace image data and the retrace image data described above in order to create an accurate sample surface image even when there is a steep change in the elevation of the sample surface. Specifically, the trace image data and the retrace image data obtained by scanning the same region are compared, and a part in which the data should be replaced is automatically recognized, for example, in the trace image data (or in the retrace image data), and data processing is carried out to replace the data of that part with the retrace image data (or trace image data). Also, Patent Literature 1 proposes another technique in which a part where the value of the trace image data (or retrace image data) exceeds a predetermined threshold is detected, and the data of that part is replaced with the retrace image data (or trace image data).
However, such a data processing technique raises the following problem. In Patent Literature 1, when a steep protrusion, for example, is present on the sample surface, the trace image data and the retrace image data are compared to choose the less accurate image data on the assumption that the feedback control is tracked properly on an upward slope prior to the apex part of the protrusion, and the feedback control is disturbed after passing the apex part, that is, after the upward slope is sharply changed to a downward slope. However, the assumption may not be always true depending on the shape of the protrusion, such as for a protrusion having one side steep and the other side moderate, or a protrusion having a trapezoid form with an almost flat top. For this reason, there are cases in which the less accurate one of the trace image data and the retrace image data may be selected, so that an accurate sample surface image may not be obtained.
Also, when a value of the trace image data exceeds a predetermined threshold in a certain part, it is not always true that the retrace image data obtained in that part represents the sample surface shape more accurately. Accordingly, even when said another technique disclosed in Patent Literature 1 is used, there are cases in which the less accurate one of the trace image data and the retrace image data may be selected, so that an accurate sample surface image may not be obtained, either.
Patent Literature 1: JP 2006-105684 A